If I have two bodies excatly next to each other, how to make sure that they are really in contact, i.e. how to ensure that the two bodies are really in contact with one sharing interface?

And how we can keep two bodies in contact as two separate bodies of one single part, without merging and forming single body? Do I need to keep them as freezing bodies or is there any other method to keep them separated (or sliced)

Yes you use freeze or add frozen to keep them from merging. To make sure they are in contact, model them that way, but even if they are not in perfect contact you can still apply interfaces between them if you are using CFX (and perhaps other programs too)